Chorus guitar pedals are one of the most essential and popular pedals for guitarists. A chorus pedal splits your guitar signal into two parts and detunes and delays one of them. It can be used to add subtle modulations to your sound or using in more radical ways for more drastic effects and widening your sound.
Chorus is one of a few types of modulation effect, it should be placed fairly late in your pedal chain. It should come after a wah pedal, compression pedal, overdrive pedal, and distortion pedal, but before your delay pedal, tremolo pedal, or reverb pedal.
Its a pedal you might just want to put in your guitar amp fx loop.
Two songs to hear a chorus; The Police with Walking On The Moon and Nirvana: Come As You Are, sounds great clean or with lots of drive.